Process for determining the nature of the subsoil by the aid of electricity.



c. SCHLUMBEHGER.

PROCESS FOR DETERMINING THE NATURE OF THE SUBSOIL BY THE AID 0F ELECTRICITY.

APPLICATION FILED JAN. 2. 191a.

1,163,468. I Patented new, 1915.

2 SHEETHHEET I g K m.

' c. SCHLUMBERGER'. I PROCESS FOR DETERMINING THE NATURE OF THE SUBSOIL BY THE AID 0F ELECTRICITY.

APPLICATION FILED JAN. 2. 1913- Patented Dec. 7

TZ W lxhhqcoow CONRAD SCHLUMBERGER, OF PARIS, FRANCE.

PROCESS FOR- DETERMINING THE NATURE OF THE SUBSOIL BY AID OF ELECTRICITY.

Specification of Letters Patent.

Patented Dec. 7, 19115.

- Application filed January 2, 1913. Serial No. 739,728.

To all whom it may concern Be it known that I, CONRAD SCHLUM- BERGER, of 7 Rue Las Cases, Paris, France,

engineer, have invented a new and useful Improvement in Processes for Determining the Nature of the Subsoil by the Aid of Electricity, which improvement is fully set forth in the following specification.

The purpose of the present invention is to provide a method whereby to determine the natureof the sub-soil of a tract of land and to detect and localize minerals, or deposits, stone laye'rs, and the like', below the surface, without the necessity of digging or excavating.

For this purpose the invention consists 1n the steps, mode of proceeding, and features now to be describe The accompanying drawings illustrate more or less diagrammatically the method of the present invention as well as apparatus :iutable for carrying out the same in prac- ICe.

In these drawings Figure 1 is a diagram showing a typical arrangement of the apparatus in practice; Fig. 2 is a diagramillustrating the method of locatingv an equipotential curve; Fig. 3 is a view, partlyin sec tion, of a type of electrode used in carrying out the methodof the invention; Fig. 4 illustrates diagrammatically the method of determining the'relative potential of points on an equipotential curve, and Figs. 5 and,

6 illustrate in vertical section and plan, re-. spectlvely, a typical distribution. of equipotential surface and fcurvesjn a 'speclfic instance.

In the first place, two points and B on the surface of the ground are selected, and a constant or variable difference of potential is applied between them. A series of equipotential"curves are then plotted around the said points A and B. The chart of equipotential curves so obtained, which may be likened to a topographical chart provided with curves of equal altitude, will serve to determine the distribution of the electric current in the interior of the earth between the points A and B, so far at least as this is possible by observations at the Surface. The chart will, in particular, furnish .important data for determining the heterogeneities of the sub-soil with relation the, geologicalv nature of the said sub-soil, whether the heterogeneous character of the same be due to electrolytic causes, such as variations in the water or moisture contained in the land,-or to the presence of metals or conducting minerals such as magnetite, pyrites, galena, or other conducting material.

In order to apply a difference of potential between the points A and B of the ground (see Fig. 1), a suitable source D of electricity is employed, such, for example, as a continuous current dynamo-machine. The poles of this source of electricity are connected by a an insulated conductor L with earth terminals arranged at the points A and B. Each of these earth terminals or leads may consist, for example, of a group of metal pegs, or stakes, which are 'driven into the ground and connected with the conductor L.

In order to determine and plot the curves I ground at a series of different points P, Q,

R, etc., the galvanometer being inspected each time. The galvanometer will show a deviation under the influence of the difference of potentialapplied between A and B each time that the point of the ground in contact with the electrode 6 has not the same potential as the point M at which the other electrode e is grounded. When, however, the electrode 6 has been broughtinto con-. tact with a point of the ground having the same potential asthe point M, no deviation of the galvaiiometer occurs. In Fig. 2, it is assumed that the point Q located in this ,manner is of equal potential with the point M, said point Q lying between the points P and R, which show opposite deviations v on the galvanometer'. In the same manner a Sllfi'lClGIlt number of further points having the same potential are located, for example the points Q, Q". By plotting all the points having the same potential upon a map of the region in question, and connect ing them by a line, the corresponding equipotential curve will be obtained. In carrying out this method in practice, a number of disturbing-causes must be taken into consideration For example, if two ordinary electrodes consistingof two metal spikes or stakes are'driven into the soil and are connected by a conductor, a galvanic element is formed in which the moist or wet ground takes the part of the electrolyte. The electro-motive force of such an element is ordinarilyin the neighborhood of a few tenths of a-volt, because the metals of both stakes are never exactly identical in character.

Moreover, th1s electromotive force Wlll of.

course be very variable, sincethe least current traversing the circuit produces the polarization of the stakes by reason of electrolysis.

For this reason, and under these circumstances, it is notpossible to introduce a sensitive galvanometer into the conductor without obtaining a considerable deviation of the needle and'even subjecting the same to injury. It is, therefore, impossible to observe small potential differences on the surface of the earth with the arrangement above set forth. In order to remove this diflicu-lty I construct and arrange theelectrodese-e in a special manner. As shown in the drawing, Fig. 3, each of these electrodes consists of arod a of copper which extends into a concentrated solution b of copper-sulfate, which is contained ina porous receptacle 6; This porous'receptacle is the onlypart of the electrode which comes into-contact with earth. Manifestly the rod forming the electrode may be made of other metalthan copper, and may extend into a solution of a suitable salt of the metal of which it is formed. With the electrode formed in this manner it is possible to decrease the 'electro motive force of the galvanic element formed by the two electrodes,

- earth currents which are continually 'passing through the earths' crust and which also cause the galvanometer to deviate even when no difference of potential is applied between the points A and B, such deviations increasa ing with the length of the conductor Z. The

errors of observation due to such earth cur- 1 rents, as well as to the residual slight electromotive force between the electrodes 6 e, are eliminated by periodically reversing the difference of potential between the points A and B, and reading off on the galvanometer only these deviations which are due to such tween the different curves.

4 units.

will be obtained by ex ressing the drrzlp 0 100,000 ohms, for exam le.

reversals, and which are always readily distinguishable from the deviations, due to earth currents, by virtue of their regular occurrence.

To be able to give to the equipotential curves their correct and complete signifithe potential drop between any two given;

curves of equal potential on the chart, is always more or less variable'during the experiment; so that it would be inaccurate to indicate these potential drops in absolute quantities, for example, in volts. For practical purposes, it isentirely suflicient to know merely the relative drops of potential be- In order "to measure these relative drops of potential, the method indicatedin Fig. 4 may be employed. The potential of an equipotential curve designated a is designated as 100, for example, and the potential of another equipotential curve (5 at a considerable distance from the point A is given the arbitrary value, zero. The drop in potential between the curves a and {5 therefore amounts to 100 arbitrary If we now take an equipotential curve 7, situated between the equipotential curves 0: and L), the number to be given it of potential between y an (5 in the units a pted for expressing the drop 10f potential between oz and {5. Thisnumber may be obtained by a method of measurement, in which an arrangement in the manner of a Wheatstone bridge, is employed. Between the two equipotential curves as and (5 is introduced a conductor containing a high resistance B,

This bridge carries a derived current rom a to 0 the potential along this conductor decreasing in 'proportion to the resistance. A point on the equipotential curve 7 is then connected with a point at on the resistanceR, a galvanometer 9 being introduced into the conductor Z connecting the two points. The extremity of the conductor Z at the point mis'then moved along the resistance R until the galvanometer indicates zero, which means that Y and :0 have the same potential. For example, assuming that for this position of the point 00 the 100,000 ohms of the resistance are divided in such a manner that a resistance of 35,000 ohms exists between the point a and the equipotential curvefi, the 7 scribed when employing this method, the non-polarizable electrodes described above must be emjployedand the deviations of the galvanometer must be read off by employing the method of reversal of current between the points A and B as above described. With the aid of the chart of potentials thus obtained, deductions may be made with regard to the interior conductivity of the region represented on the chart, and therefrom with regard to the geological nature and structure of the sub-soil thereof. A potential chart thus plotted-is independent, firstof the mean specific conductivity of theground, secondly, of the absolute value of the difference of potential applied between the points A and B, which may be constant or variable,

v and, thirdly, of the nature of the connec- ,tiVe mass.

tions to earth in A and B. This is true as well for the form of equipotential curves as for their numbering, so that the potential chart of the region in the vicinity of the points A and B is entirely independent of the character or the behavior of the meas-, uring apparatus, and of all experimental contingencies. The equipotential curves are the lines of intersection of the surface of the earth with the equipotential surfaces extending into the interior of the earth. Where the ground is homogeneous these surfaces may be ascertained by calculation, wherebyit is possible to obtain the equipotential curves for the surface by such theoretical calculations in the case of homogeneous territory. If we assume the points A and B to be located at a considerable distance from each other, as will be the case in practice, it will betrue that the equipotential curves in the vicinity of A .or B will approximate very closely to circles for homogeneous ground, said circles being described around the points A or B as their centers. The equipotential surfaces extending into the earth will accordingly approximate very closely to hemispheres.

Now, if it is assumed that the ground is not homogeneous,- but contains a conductive mass Z near the point A, (see Figs. 5 and 6) .the equipotential surfaces will not be hemi-.

spheres, but will have deviations or defor-' mations in the neighborhood of the conduc-- These deformations or distortions of the equipotential surfaces will naturally be present also in the equipotential curves at the surface. The deviations or distortions of the equipotential curves at the surface, as a rule are of such a character that these curves are reentrant or concave over the point of the conductive mass which is nearest the point A, while the equipotentialm curves lying over the opposite end of the said mass are curved outward, forming protuberances. The equipotential curves lying. over' the middle of such mass are however deformed to a relatively small extent. This condition of the equipotential curves is represented in Figs. 5 and 6, of the drawing. It is hence clear that the form of the equipotential curves determined by the. above method furnishes means whereby to readily ascertain-the position and the extent of a conductive mass in the ground. The depth masses will influence only curves of a great radius.

For carrying out the method above set forth, it is suflicient to employ a plant or apparatus of normalefliciency and such as can readily'be obtained on the market. For example, in order to produce the potential difference, (required, between the points a and {5, a dynamo machine from one to two.

kilowatt power furnishing a continuous current of from 200 to 400 volt will be efi'ective. What I claim and desire to secure by Letters Patent, is:

1. The process of determining the nature of the sub-soil, which consists in maintaining a difference of potential between two points on'the surface of a given tract, plotting a chart indicating the equipotential curves with relation to the two points, and then comparing the equipotential curves thus obtained with the curves of an equipotential chart calculated with reference to the said two points for homogeneous soil.

' 2. The process ofdetermining the nature of the sub-soil, which consists in maintaining a difference of potential between two points on the surface of a given tract, periodically reversing the current betweenthe said two points to eliminate disturbances by earth-currents or polarization at the electrodes, plotting a chart indicating the equipotential curves with relation; to the two points and then comparing theequipotential curves thus obtained with the curves of an equipotential chart calculated with reference to the said two points for homogeneous soil.

3. The process of determining the nature of the sub-soil, which consists in maintaining a. difference of potential between two points on the surface of a given tract, plotting a chart indicating the equipotential curvesin the vicinity of the two points, measuring the relative drop of potential between the different equipotential curves,

then comparing. the curves and figures thus obtained with the theoretical equipotential curves and figures calculated for hoinogeneous soil.

LApparatus for determining the nature of the sub-soil comprising a pair of unpolarizable electrodes connected by an insulated line containing a galvanometer.

5. Apparatus for determining the nature of the sub-soil comprising a pair of unpolarizable electrodes connected by an insulated line containing a galvanometer, each of said electrodes comprisinga rod of substantially pure metal connected to said line, and a porous receptacle containing a solution of a salt of said metal into which said rod dips, said porous receptacle being arranged to prevent contact of said rod with earth.

6. In apparatus for determining the nature of the sub-soil, an electrode'comprising a rod of metal and a porous receptacle con-' taining a solution of a salt of said metal into which said rod dips, said porous receptacle being arranged to prevent contact of said rod with earth.

In testimony whereof I have signed this specification in the presence of two subscribing Witnesses.

CONRAD SGHLUMBERGER/ Witnesses:

LUCIEN MEMMINGER, Pkfimvi HARTIE. 

